Audio level indicator



Nov. 20, 1951 R. G. NUCKOLLS 2,575,358

AUDIO LEVEL INDICATOR Filed Aug. 9, 1946 .IO\ I INPUT o VARIABLE l2 GAIN AMPLIFIER ELIE-"=5 jo VARIABLE ls GAIN AMPLIFIER 3mm RICHARD G. N UCKOLLS Patented Nov. 20, 1951 AUDIO LEVEL INDICATOR Richard G. Nuckolls, Grinnell, Iowa Application August 9, 1946, Serial No. 689,352

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 2 Claims.

This invention relates to an indicating circuit and'more particularly to a circuit for indicating the amplitude level of an audio frequency signal.

It is an object of this invention to provide a circuit which will give an audible indication of any signal above a preselected amplitude.

It is a further object of this invention to provide a circuit for comparing the peak amplitude level of different audio frequency signals.

Other objects and advantages of the invention will' be apparent from the following detailed description when taken in conjunction with the accompanying drawings which illustrate .preferred embodiments of the invention, and in which:

Fig. 1 is a circuit diagram showing one embodiment of the invention, and

Fig. 2 is a circuit diagram of another embodiment of the invention.

In accordance with the present invention, a circuit is contemplated which will allow the amplitude levels of all signals to be compared. Additionally, the circuit may be arranged 'such that a minimum level of amplitude measurement or comparison is established and only signalshav; ing an amplitude greater than this minimum level will be able to be measured or compared. A voltage breakdown device, such as a neon bulb is proposed as the means whereby the above men-' tioned minimum amplitude level may be established. For voltages below a certain value, the neon bulb will not conduct, but for signals of greater amplitude a surge current will flow through the bulb and will cause an audible signal to be heard in a pair of headphones which are connected in parallel with the bulb. By placing a power supply having a variable voltage output in series with the bulb, the amplitude level of the signal necessary to cause a surge current to flow through the bulb may be adjusted.

In Fig. l, to which reference is now had, there is shown an embodiment of the invention, comprising a variable-gain amplifier ID, a neon bulb II connected to the output of the amplifier ID, a pair of phones I2 and a power supply I3 which extends from the low side of the neonv bulbj'lII to ground. A suitable resistance I4 is connected in series with the neon bulb to limit the current through the neon bulb and the headphones I2 are in parallel with the combination of the neon bulb I I and the resistance I4. Assume now that an audio frequency signal is introduced to .the input I9 of the amplifier III, which will be maintained at constant gain. If the peak amplitude of the signal is not sufiiciently large, tube II will not conduct. However, a completed path will be presented to the signal through the amplifier Ill and the headphones I2, thus allowing the sig-. nal to be heard in the headphones I2. If, however, the signal introduced'to the input of the amplifier I0 is sufliciently large to cause the neon bulb I I to conduct, the resulting surge current will produce a surge voltage across the resistance I4 and neon bulb II, which voltage will also appear across the headphones l2 and cause a signal having a characteristic popping noise to be heard in the headphones. By placing the power supply I3 in series with the neon bulb II and the resistance I4, the amplitudeof the signal required to break down the bulb II may be varied, depending upon the value of the voltage from the supply I3. Additionally, the gain of the amplifier It may be varied for each signal by means of the gain control I1 such that the signal will be barely audible in the headphones I2. In this way, the peak amplitude of different signals may be compared by noting the position of the variable gain control for each signal.

Fig. 2 shows an alternate embodiment of the invention. In addition to the variable gain amplifier Ill, the neon bulb II, the headphones I2 and the power supply I3, a matching transformer I5 and a step-up trans-former I6 are also included. The output from the variable gain amplifier is introduced to the primary windings of the transformers I5 and I6, both of which windings are in parallel. The secondary'of the transformer I5 is connected to the headphones I2, while the neon bulb II serves as the load for the secondary of the transformer I 6 and is connected from one side of the transformer I6 secondary to ground. One side of the primary and secondary windings of transformer I6 are connected together and to the power supply I3. As with the circuit shown in Fig. l, a signal having an insufficient peak amplitude to break down the neon bulb II will still be heard in the headphone I2. If, however, the signal is sufficiently strong to cause the bulb II to conduct, the resultin current will flow in the windings of the transformer IB. Thus, a resultant voltage will appear across the primary winding of the transformer I6 and this same voltage will appear across the primary winding of the transformer I5, since the two primary windings are in parallel. A surge current of sufilcient strength to cause an audible sound similar to the surge of noise described in connection with Fig. l to emanate from the headphones will now be obtained. As in Fig. 1, the minimum amplitude level of the surge signal which will be heard in the headphones will be made dependent upon the value of voltage from the power supply l3. In Fig. 1, an increase in the value of the positive voltage supplied to the tube II from the tap on supply (3 necessitates a signal of greater peak amplitude to cause conduction. In Fig. 2, an increase in the value of the positive voltage supplied to the tube ll from the tap on supply l3 will permit conduction by tube II for a smaller peak signal amplitude. Additionally, the circuit of Fig. 2 is simi lar to that shown in Fig. l in that the gain of the amplifier l0 may be varied by means of the gain control 18 for each signal such thatthe surge signal will be barely audible in the headphones l2. Inthis way, the peak -amplitude of different. sig-. nals may be compared by noting the position of the variable gain control for each signal.

While certain preferred embodiments of this invention have been described, it is realized that many modifications and variations of this invention may be made and no limitations upon this invention are intended other than may be imposed by the. scope of the appended claims.

The invention described herein may be manufactured and used by or for the Government of the. United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. A circuit for indicating the amplitude level of an audio frequency signal comprising, an amplifier, a voltage breakdown means connected to the output of said: amplifier said last named means being arranged to conduct in response to signals having an amplitude greater than a specified amplitude, and an audio reproducing means coupled in parallel across said last named means whereby conduction by said voltage breakdown means may be detected.

2. A circuit for indicating the amplitude level of an audio frequency signal comprising an amplifier, a transformer having a primary and secondary winding with two terminals on each winding, a power supply connected to a terminal of the primary and secondary windings, the second terminal of said primary winding connected to the. output of. said amplifier stage, a neon bulb REFERENCES CITED Thefollowing references are of record in the, file of this patent:

UNITED STATES PATENTS Number Name Date 1,635,779 Carter July 12,- 1927 2,128,395' Berndt et al Aug. 30', 1938 2,394,573 Walsh et a1. Feb 12, 1946 2,395,368 Bull Feb. 19', 1946 

